Targeting the proper amyloid-beta neuronal toxins: a path forward for Alzheimer's disease immunotherapeutics

The contribution of biophysical and structural studies of protein self-assembly to the design of therapeutic strategies for amyloid diseases

Many neurodegenerative disorders, including Alzheimer's, Parkinson's and the prion diseases, are characterized by a conformational conversion of normally soluble proteins or peptides into pathological species, by a process of misfolding and self-assembly that leads ultimately to the formation of amyloid fibrils. Recent studies support the idea that multiple intermediate species with a wide variety of degrees of neuronal toxicity are generated during such processes. The development of a high level of knowledge of the nature and structure of the pathogenic amyloid species would significantly enhance efforts to underline the molecular origins of these disorders and also to develop both accurate diagnoses and effective therapeutic interventions for these types of conditions. In this review, we discuss recent biophysical and structural information concerning different types of amyloid aggregates and the way in which such information can guide rational therapeutic approaches designed to target specific pathogenic events that occur during the development of these highly debilitating and increasingly common diseases.

Newly Developed Drugs for Alzheimer's Disease in Relation to Energy Metabolism, Cholinergic and Monoaminergic Neurotransmission

Current options for Alzheimer's disease (AD) treatment are based on administration of cholinesterase inhibitors (donepezil, rivastigmine, galantamine) and/or memantine, acting as an N-methyl-D-aspartate (NMDA). Therapeutic approaches vary and include novel cholinesterase inhibitors, modulators of NMDA receptors, monoamine oxidase (MAO) inhibitors, immunotherapeutics, modulators of mitochondrial permeability transition pores (mPTP), amyloid-beta binding alcohol dehydrogenase (ABAD) modulators, antioxidant agents, etc. The novel trends of AD therapy are focused on multiple targeted ligands, where mostly ChE inhibition is combined with additional biological properties, positively affecting neuronal energy metabolism as well as mitochondrial functions, and possessing antioxidant properties. The present review summarizes newly developed drugs targeting cholinesterase and MAO, as well as drugs affecting mitochondrial functions.

Unmodified and pyroglutamylated amyloid β peptides form hypertoxic hetero-oligomers of unique secondary structure

Amyloid β (Aβ) peptide plays a major role in Alzheimer's disease (AD) and occurs in multiple forms, including pyroglutamylated Aβ (AβpE). Identification and characterization of the most cytotoxic Aβ species is necessary for advancement in AD diagnostics and therapeutics. While in brain tissue multiple Aβ species act in combination, structure/toxicity studies and immunotherapy trials have been focused on individual forms of Aβ. As a result, the molecular composition and the structural features of "toxic Aβ oligomers" have remained unresolved. Here, we have used a novel approach, hydration from gas phase coupled with isotope-edited Fourier transform infrared (FTIR) spectroscopy, to identify the prefibrillar assemblies formed by Aβ and AβpE and to resolve the structures of both peptides in combination. The peptides form unusual β-sheet oligomers stabilized by intramolecular H-bonding as opposed to intermolecular H-bonding in the fibrils. Time-dependent morphological changes in peptide assemblies have been visualized by atomic force microscopy. Aβ/AβpE hetero-oligomers exert unsurpassed cytotoxic effect on PC12 cells as compared to oligomers of individual peptides or fibrils. These findings lead to a novel concept that Aβ/AβpE hetero-oligomers, not just Aβ or AβpE oligomers, constitute the main neurotoxic conformation. The hetero-oligomers thus present a new biomarker that may be targeted for development of more efficient diagnostic and immunotherapeutic strategies to combat AD.

Investigation of the Subcellular Neurotoxicity of Amyloid-β Using a Device Integrating Microfluidic Perfusion and Chemotactic Guidance

Alzheimer's disease (AD) is a neurodegenerative disorder with the histopathological hallmark of extracellular accumulation of amyloid-β (Aβ) peptide in brain senile plaques. Though many studies have shown the neural toxicity from various forms of Aβ peptides, the subcellular mechanisms of Aβ peptide are still not well understood, partially due to the technical challenges of isolating axons or dendrites from the cell body for localized investigation. In this study, the subcellular toxicity and localization of Aβ peptides are investigated by utilizing a microfluidic compartmentalized device, which combines physical restriction and chemotactic guidance to enable the isolation of axons and dendrites for localized pharmacological studies. It is found that Aβ peptides induced neuronal death is mostly resulted from Aβ treatment at cell body or axonal processes, but not at dendritic neurites. Simply applying Aβ to axons alone induces significant hyperactive spiking activity. Dynamic transport of Aβ aggregates is only observed between axon terminal and cell body. In addition to differential cellular uptake, more Aβ-peptide secretion is detected significantly from axons than from dendritic side. These results clearly demonstrate the existence of a localized mechanism in Aβ-induced neurotoxicity, and can potentially benefit the development of new therapeutic strategies for AD.

Vincamine Alleviates Amyloid-β 25-35 Peptides-induced Cytotoxicity in PC12 Cells

Objective:

Vincamine is a plant alkaloid used clinically as a peripheral vasodilator that increases cerebral blood flow and oxygen and glucose utilization by neural tissue to combat the effect of aging. The main purpose of the present study is to investigate the influence of vincamine on amyloid-β 25–35 (Aβ25–35) induced cytotoxicity, to gain a better understanding of the neuroprotective effects of this clinically used anti-Alzheimer's disease drug.

Materials and Methods:

Oxidative stress was assessed by measuring malondialdehyde, glutathione, and superoxide dismutase (SOD) levels. Cell viability was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Cell apoptosis detection was performed using an Annexin-V-FITC Apoptosis Detection Kit. The production of reactive oxygen species (ROS) was determined using an ROS Assay Kit. Western blot detection was carried out to detect the protein expression.

Results:

Our studies showed that pretreatment with vincamine could reduce Aβ25–35 induced oxidative stress. Vincamine markedly inhibited cell apoptosis dose-dependently. More importantly, vincamine increased the phosphatidylinositol-3 kinase (PI3K)/Akt and Bcl-2 family protein ratios on preincubation with cells for 2 h.

Conclusion:

Above observation led us to assume that one possible mechanism of vincamine protects Aβ25-35-induced cell death could be through upregulation of SOD and activation of the PI3K/Akt pathway.

SUMMARY

Vincamine ameliorates amyloid-β 25–35 (Aβ25–35) peptides induced cytotoxicity in PC12 cells

Vincamine reduces Aβ 25–35 peptides induced apoptosis of PC12 cells

Vincamine activates the phosphatidylinositol-3 kinase/Akt pathway

Vincamine up-regulates the superoxide dismutase.

Abbreviation used: Aβ25-35: Amyloid-β 25-35; AD: Alzheimer's disease; BCA: Bicinchoninic acid; GSH: glutathione; PBS: Phosphate buffered solution; SDS: Sodium dodecylsulphate; SOD: Superoxide dismutase

The Proteasome and Oxidative Stress in Alzheimer's Disease

SIGNIFICANCE

Alzheimer's disease is a neurodegenerative disorder that is projected to exceed more than 100 million cases worldwide by 2050. Aging is considered the primary risk factor for some 90% of Alzheimer's cases but a significant 10% of patients suffer from aggressive, early-onset forms of the disease. There is currently no effective Alzheimer's treatment and this, coupled with a growing aging population, highlights the necessity to understand the mechanism(s) of disease initiation and propagation. A major hallmark of Alzheimer's disease pathology is the accumulation of amyloid-β (Aβ) aggregates (an early marker of Alzheimer's disease), and neurofibrillary tangles, comprising the hyper-phosphorylated microtubule-associated protein Tau. Recent Advances: Protein oxidation is frequently invoked as a potential factor in the progression of Alzheimer's disease; however, whether it is a cause or a consequence of the pathology is still being debated. The Proteasome complex is a major regulator of intracellular protein quality control and an essential proteolytic enzyme for the processing of both Aβ and Tau. Recent studies have indicated that both protein oxidation and excessive phosphorylation may limit Proteasomal processing of Aβ and Tau in Alzheimer's disease.

CRITICAL ISSUES

Thus, the Proteasome may be a key factor in understanding the development of Alzheimer's disease pathology; however, its significance is still very much under investigation.

FUTURE DIRECTIONS

Discovering how the proteasome is affected, regulated, or dysregulated in Alzheimer's disease could be a valuable tool in the efforts to understand and, ultimately, eradicate the disease. Antioxid. Redox Signal. 25, 886-901.

Brain amyloid in preclinical Alzheimer's disease is associated with increased driving risk

Introduction

Postmortem studies suggest that fibrillar brain amyloid places people at higher risk for hazardous driving in the preclinical stage of Alzheimer's disease (AD).

Methods

We administered driving questionnaires to 104 older drivers (19 AD, 24 mild cognitive impairment, and 61 cognitive normal) who had a recent ¹⁸F-florbetapir positron emission tomography scan. We examined associations of amyloid standardized uptake value ratios with driving behaviors: traffic violations or accidents in the past 3 years.

Results

The frequency of violations or accidents was curvilinear with respect to standardized uptake value ratios, peaking around a value of 1.1 (model r² = 0.10, P = .002); moreover, this relationship was evident for the cognitively normal participants.

Discussion

We found that driving risk is strongly related to accumulating amyloid on positron emission tomography, and that this trend is evident in the preclinical stage of AD. Brain amyloid burden may in part explain the increased crash risk reported in older adults.

Copper Ion Binding Site in β-Amyloid Peptide

β-Amyloid aggregates in the brain play critical roles in Alzheimer's disease, a chronic neurodegenerative condition. Amyloid-associated metal ions, particularly zinc and copper ions, have been implicated in disease pathogenesis. Despite the importance of such ions, the binding sites on the β-amyloid peptide remain poorly understood. In this study, we use scanning tunneling microscopy, circular dichroism, and surface-enhanced Raman spectroscopy to probe the interactions between Cu²⁺ ions and a key β-amyloid peptide fragment, consisting of the first 16 amino acids, and define the copper-peptide binding site. We observe that in the presence of Cu²⁺, this peptide fragment forms β-sheets, not seen without the metal ion. By imaging with scanning tunneling microscopy, we are able to identify the binding site, which involves two histidine residues, His13 and His14. We conclude that the binding of copper to these residues creates an interstrand histidine brace, which enables the formation of β-sheets.

A Mitocentric View of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease with an increasing morbidity, mortality, and economic cost. Plaques formed by amyloid beta peptide (Aβ) and neurofibrillary tangles formed by microtubule-associated protein tau are two main characters of AD. Though previous studies have focused on Aβ and tau and got some progressions on their toxicity mechanisms, no significantly effective treatments targeting the Aβ and tau have been found. However, it is worth noting that mounting evidences showed that mitochondrial dysfunction is an early event during the process of AD pathologic changes. What is more, these studies also showed an obvious association between mitochondrial dysfunction and Aβ/tau toxicity. Furthermore, both genetic and environmental factors may increase the oxidative stress and the mitochondria are also the sensitive target of ROS, which may form a vicious feedback between mitochondrial dysfunction and oxidative stress, eventually resulting in deficient energy, synaptic failure, and cell death. This article reviews the previous related studies from different aspects and concludes the critical roles of mitochondrial dysfunction in AD, suggesting a different route to AD therapy, which may guide the research and treatment direction.

The Role of Amyloid-β Oligomers in Toxicity, Propagation, and Immunotherapy

The incidence of Alzheimer's disease (AD) is growing every day and finding an effective treatment is becoming more vital. Amyloid-β (Aβ) has been the focus of research for several decades. The recent shift in the Aβ cascade hypothesis from all Aβ to small soluble oligomeric intermediates is directing the search for therapeutics towards the toxic mediators of the disease. Targeting the most toxic oligomers may prove to be an effective treatment by preventing their spread. Specific targeting of oligomers has been shown to protect cognition in rodent models. Additionally, the heterogeneity of research on Aβ oligomers may seem contradictory until size and conformation are taken into account. In this review, we will discuss Aβ oligomers and their toxicity in relation to size and conformation as well as their influence on inflammation and the potential of Aβ oligomer immunotherapy.

Antibody-based PET imaging of amyloid beta in mouse models of Alzheimer's disease

Owing to their specificity and high-affinity binding, monoclonal antibodies have potential as positron emission tomography (PET) radioligands and are currently used to image various targets in peripheral organs. However, in the central nervous system, antibody uptake is limited by the blood-brain barrier (BBB). Here we present a PET ligand to be used for diagnosis and evaluation of treatment effects in Alzheimer's disease. The amyloid β (Aβ) antibody mAb158 is radiolabelled and conjugated to a transferrin receptor antibody to enable receptor-mediated transcytosis across the BBB. PET imaging of two different mouse models with Aβ pathology clearly visualize Aβ in the brain. The PET signal increases with age and correlates closely with brain Aβ levels. Thus, we demonstrate that antibody-based PET ligands can be successfully used for brain imaging.

Naturally occurring autoantibodies against Aβ oligomers exhibited more beneficial effects in the treatment of mouse model of Alzheimer's disease than intravenous immunoglobulin

Alzheimer's disease (AD) is characterized by memory loss, intracellular neurofibrillary tangles, and extracellular plaque deposits composed of β-amyloid (Aβ). Previous reports showed that naturally occurring autoantibodies, such as intravenous immunoglobulin (IVIG), benefited patients with moderate-stage AD who carried an APOE-ε4 allele. However, the mechanism underlying the role of IVIG remains unclear. In this study, we identified naturally occurring autoantibodies against Aβ oligomers (NAbs-Aβo), which were purified by Aβ42 oligomer or Cibacron Blue affinity chromatography from IVIG and termed as Oli-NAbs and Blue-NAbs, respectively. Oli-NAbs and Blue-NAbs recognized Aβ42 oligomers or both Aβ40 and 42 oligomers, differently. Both antibodies inhibited Aβ42 aggregation and attenuated Aβ42-induced cytotoxicity. Compared with vehicles, Oli-NAbs, Blue-NAbs and IVIG significantly improved the memory and cognition, and reduced the soluble and oligomeric Aβ levels in APPswe/PS1dE9 transgenic mice. Further investigation showed that Blue-NAbs at increased doses effectively decreased plaque burden and insoluble Aβ levels, whereas Oli-NAbs significantly declined the microgliosis and astrogliosis, as well as the production of proinflammatory cytokines in vivo. Therefore, high levels of these antibodies against oligomeric Aβ40 or Aβ42 were required, correspondingly, to achieve the optimal effect. NAbs-Aβo could be condensed to a high concentration by affinity chromatography and its isolation from IVIG may not interfere with the normal function of conventional IVIG as its concentration is very low. Thus, the isolated NAbs-Aβo as an extra product of plasma required low cost and the enriched NAbs-Aβo may be more feasible than IVIG for the treatment of AD.

β-Amyloid induces nuclear protease-mediated lamin fragmentation independent of caspase activation

β-Amyloid (Aβ), a hallmark peptide of Alzheimer's disease, induces both caspase-dependent apoptosis and non-apoptotic cell death. In this study, we examined caspase-independent non-apoptotic cell death preceding caspase activation in Aβ42-treated cells. We first determined the optimal treatment conditions for inducing cell death without caspase activation and selected a double-treatment method involving the incubation of cells with Aβ42 for 4 and 6 h (4+6 h sample). We observed that levels of lamin A (LA) and lamin B (LB) were reduced in the 4+6 h samples. This reduction was decreased by treatment with suc-AAPF-CMK, an inhibitor of nuclear scaffold (NS) protease, but not by treatment with z-VAD-FMK, a pan-caspase inhibitor. In addition, suc-AAPF-CMK decreased the changes in nuclear morphology observed in cells in the 4+6 h samples, which were different from nuclear fragmentation observed in STS-treated cells. Furthermore, suc-AAPF-CMK inhibited cell death in the 4+6 h samples. LA and LB fragmentation occurred in the isolated nuclei and was also inhibited by suc-AAPF-CMK. Together, these data indicated that the fragmentation of LA and LB in the Aβ42-treated cells was induced by an NS protease, whose identity is not clearly determined yet. A correlation between Aβ42 toxicity and the lamin fragmentation by NS protease suggests that inhibition of the protease could be an effective method for controlling the pathological process of AD.

Amyloid-β Receptors: The Good, the Bad, and the Prion Protein

Several different receptor proteins have been identified that bind monomeric, oligomeric, or fibrillar forms of amyloid-β (Aβ). "Good" receptors internalize Aβ or promote its transcytosis out of the brain, whereas "bad" receptors bind oligomeric forms of Aβ that are largely responsible for the synapticloss, memory impairments, and neurotoxicity that underlie Alzheimer disease. The prion protein both removes Aβ from the brain and transduces the toxic actions of Aβ. The clustering of distinct receptors in cell surface signaling platforms likely underlies the actions of distinct oligomeric species of Aβ. These Aβ receptor-signaling platforms provide opportunities for therapeutic intervention in Alzheimer disease.

Single molecule experiments emphasize GM1 as a key player of the different cytotoxicity of structurally distinct Aβ1-42 oligomers

It is well established that cytotoxic Aβ oligomers are the key factor that triggers the initial tissue and cell modifications eventually culminating in the development of Alzheimer's disease. Aβ1-42 oligomers display a high degree of polymorphism, and several structurally different oligomers have been described. Amongst them, two types, recently classified as A+ and A-, have been shown to possess similar size but distinct toxic properties, as a consequence of their biophysical and structural differences. Here, we have investigated by means of single molecule tracking the oligomer mobility on the plasma membrane of living neuroblastoma cells and the interaction with the ganglioside GM1, a component of membrane rafts. We have found that A+ and A- oligomers display a similar lateral diffusion on the plasma membrane of living cells. However, only the toxic A+ oligomers appear to interact and alter the mobility of GM1. We have also studied the lateral diffusion of each kind of oligomers in cells depleted or enriched in GM1. We found that the content of GM1 influences the diffusion of both types of oligomer, although the effect of the increased levels of GM1 is higher for the A+ type. Interestingly, the content of GM1 also affects significantly the mobility of GM1 molecules themselves.

Alzheimer's Disease: Mechanism and Approach to Cell Therapy

Alzheimer’s disease (AD) is the most common form of dementia. The risk of AD increases with age. Although two of the main pathological features of AD, amyloid plaques and neurofibrillary tangles, were already recognized by Alois Alzheimer at the beginning of the 20th century, the pathogenesis of the disease remains unsettled. Therapeutic approaches targeting plaques or tangles have not yet resulted in satisfactory improvements in AD treatment. This may, in part, be due to early-onset and late-onset AD pathogenesis being underpinned by different mechanisms. Most animal models of AD are generated from gene mutations involved in early onset familial AD, accounting for only 1% of all cases, which may consequently complicate our understanding of AD mechanisms. In this article, the authors discuss the pathogenesis of AD according to the two main neuropathologies, including senescence-related mechanisms and possible treatments using stem cells, namely mesenchymal and neural stem cells.

Mitochondrial dysfunction: the missing link between aging and sporadic Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease that represents the most common form of dementia among the elderly. Despite the fact that AD was studied for decades, the underlying mechanisms that trigger this neuropathology remain unresolved. Since the onset of cognitive deficits occurs generally within the 6th decade of life, except in rare familial case, advancing age is the greatest known risk factor for AD. To unravel the pathogenesis of the disease, numerous studies use cellular and animal models based on genetic mutations found in rare early onset familial AD (FAD) cases that represent less than 1 % of AD patients. However, the underlying process that leads to FAD appears to be distinct from that which results in late-onset AD. As a genetic disorder, FAD clearly is a consequence of malfunctioning/mutated genes, while late-onset AD is more likely due to a gradual accumulation of age-related malfunction. Normal aging and AD are both marked by defects in brain metabolism and increased oxidative stress, albeit to varying degrees. Mitochondria are involved in these two phenomena by controlling cellular bioenergetics and redox homeostasis. In the present review, we compare the common features observed in both brain aging and AD, placing mitochondrial in the center of pathological events that separate normal and pathological aging. We emphasize a bioenergetic model for AD including the inverse Warburg hypothesis which postulates that AD is a consequence of mitochondrial deregulation leading to metabolic reprogramming as an initial attempt to maintain neuronal integrity. After the failure of this compensatory mechanism, bioenergetic deficits may lead to neuronal death and dementia. Thus, mitochondrial dysfunction may represent the missing link between aging and sporadic AD, and represent attractive targets against neurodegeneration.

Frailty and the risk of cognitive impairment

Aging occurs as a series of small steps, first causing cellular damage and then affecting tissues and organs. This is also true in the brain. Frailty, a state of increased risk due to accelerated deficit accumulation, is robustly a risk factor for cognitive impairment. Community-based autopsy studies show that frail individuals have brains that show multiple deficits without necessarily demonstrating cognitive impairment. These facts cast a new light on the growing number of risk factors for cognitive impairment, suggesting that, on a population basis, most health deficits can be associated with late-life cognitive impairment. The systems mechanism by which things that are bad for the body are likely to be bad for the brain can be understood like this: the burden of health deficits anywhere indicates impaired ability to withstand or repair endogenous and environmental damage. This in turn makes additional damage more likely. If true, this suggests that a life course approach to preventing cognitive impairment is desirable. Furthermore, conducting studies in highly selected, younger, healthier individuals to provide ‘proof of concept’ information is now common. This strategy might exclude the very circumstances that are required for disease expression in the people in whom dementia chiefly occurs (that is, older adults who are often in poor health).

Structure-Based Peptide Design to Modulate Amyloid Beta Aggregation and Reduce Cytotoxicity

The deposition of Aβ peptide in the brain is the key event in Alzheimer disease progression. Therefore, the prevention of Aβ self assembly into disease-associated oligomers is a logical strategy for treatment. π stacking is known to provide structural stability to many amyloids; two phenylalanine residues within the Aβ 14–23 self recognition element are in such an arrangement in many solved structures. Therefore, we targeted this structural stacking by substituting these two phenylalanine residues with their D-enantiomers. The resulting peptides were able to modulate Aβ aggregation in vitro and reduce Aβ cytotoxicity in primary neuronal cultures. Using kinetic analysis of fibril formation, electron microscopy and dynamic light scattering characterization of oligomer size distributions, we demonstrate that, in addition to altering fibril structural characteristics, these peptides can induce the formation of larger amorphous aggregates which are protective against toxic oligomers, possibly because they are able to sequester the toxic oligomers during co-incubation. Alternatively, they may alter the surface structure of the oligomers such that they can no longer interact with cells to induce toxic pathways.

Acute amnestic encephalopathy in amyloid-β oligomer-injected mice is due to their widespread diffusion in vivo

Amyloid-β (Aβ) oligomers are the suspected culprit as initiators of Alzheimer's disease (AD). However, their diffusion in the brain remains unknown. Here, we studied Aβ oligomers' dissemination and evaluated their in vivo toxicity. Wild-type mice were injected with 50 pmol of synthetic Aβ oligomers (of different size) in the hippocampus. Oligomers diffused largely in the brain as soon as 1 hour and up to 7 days after injection. A transient encephalopathy with memory impairment was induced by this unique injection. The immunoreactivity of the postsynaptic marker PSD95 was diffusely decreased. Similar results (both on memory and PSD95 immunoreactivity) were obtained with delipidated and high molecular weight oligomers (>50 kDa) but not with smaller assemblies. Tau hyperphosphorylation was observed in the oligomer-injected brains. Finally, fos immunostaining was increased in Aβ-derived diffusible ligands-injected mice, suggesting neuronal hyperactivity. Rapid and widespread diffusion of Aβ oligomers was demonstrated in vivo and associated with decreased synaptic markers and memory deficits which gives new insight to the pathogenicity of Aβ.

Amyloid β oligomers in Alzheimer's disease pathogenesis, treatment, and diagnosis

Protein aggregation is common to dozens of diseases including prionoses, diabetes, Parkinson's and Alzheimer's. Over the past 15 years, there has been a paradigm shift in understanding the structural basis for these proteinopathies. Precedent for this shift has come from investigation of soluble Aβ oligomers (AβOs), toxins now widely regarded as instigating neuron damage leading to Alzheimer's dementia. Toxic AβOs accumulate in AD brain and constitute long-lived alternatives to the disease-defining Aβ fibrils deposited in amyloid plaques. Key experiments using fibril-free AβO solutions demonstrated that while Aβ is essential for memory loss, the fibrillar Aβ in amyloid deposits is not the agent. The AD-like cellular pathologies induced by AβOs suggest their impact provides a unifying mechanism for AD pathogenesis, explaining why early stage disease is specific for memory and accounting for major facets of AD neuropathology. Alternative ideas for triggering mechanisms are being actively investigated. Some research favors insertion of AβOs into membrane, while other evidence supports ligand-like accumulation at particular synapses. Over a dozen candidate toxin receptors have been proposed. AβO binding triggers a redistribution of critical synaptic proteins and induces hyperactivity in metabotropic and ionotropic glutamate receptors. This leads to Ca(2+) overload and instigates major facets of AD neuropathology, including tau hyperphosphorylation, insulin resistance, oxidative stress, and synapse loss. Because different species of AβOs have been identified, a remaining question is which oligomer is the major pathogenic culprit. The possibility has been raised that more than one species plays a role. Despite some key unknowns, the clinical relevance of AβOs has been established, and new studies are beginning to point to co-morbidities such as diabetes and hypercholesterolemia as etiological factors. Because pathogenic AβOs appear early in the disease, they offer appealing targets for therapeutics and diagnostics. Promising therapeutic strategies include use of CNS insulin signaling enhancers to protect against the presence of toxins and elimination of the toxins through use of highly specific AβO antibodies. An AD-dependent accumulation of AβOs in CSF suggests their potential use as biomarkers and new AβO probes are opening the door to brain imaging. Overall, current evidence indicates that Aβ oligomers provide a substantive molecular basis for the cause, treatment and diagnosis of Alzheimer's disease.